Plants adapt to various types of environmental stress such as the temperature of their habitats. However, in terms of temperature stress, for example, plants are susceptible to hot or cold temperatures when exposed to environments over or under the maximum or minimum optimum growth temperature, leading to impairment upon the gradual or sudden loss of the physiological functions of cells.
Efforts have been made to expand the temperature adaptability of plants by breeding means such as selection or cross breeding in order to make use of wild plants adapted to various temperature environments for food crops, horticultural plants, and the like. The planting period in which vegetables, flowers and ornamental plants, fruit trees, and the like can be cultivated has been expanded by such breeding means as well as by protected horticulture.
Polyamines, the general term for aliphatic hydrocarbons with 2 or more primary amino groups, are ubiquitous natural substances in organisms, with more than 20 types discovered so far. Typical polyamines include putrescine, spermidine, and spermine. Known polyamine metabolism-related enzymes involved in the biosynthesis of said polyamines include arginine decarboxylase (ADC), ornithine decarboxylase (ODC), S-adenosylmethionine decarboxylase (SAMDC), spermidine synthase (SPDS), and spermine synthase (SPMS). The involvement of some of the polyamine metabolism-related enzymes in various types of environmental stress has recently been reported.
The European patent application EP 1.329.153 teaches that in plant tissues exhibiting cold stress resistance, the content of spermidine and spermine is increased. In this patent application it is exemplified that introducing the spermidine synthase gene into a plant, the spermidine and spermine levels are increased. When the transgenic plant was submitted to low temperature, it was confirmed that it has improved cold stress resistance.
The US patent application number 2006/0225154 teaches that spermidine, spermine and putrescine levels are increased when a plant is transformed with a spermidine gene synthase. This patent application states that low temperature stress defense effect can be imparted to the plant by introducing the spermidine synthase gene into the plant.
Regarding the use of the ADC gene for conferring cold stress resistance to plants, it is remarkable the fact that in the Brassicaceae family, the ADC gene appears to be duplicated, thus yielding two paralogues, generally called ADC1 and ADC2 (cf. Galoway et al., “Phylogenetic utility of the nuclear gene Arginine Decarboxylase: an example from Brassicaceae”, Molecular Biology and Evolution, 1998, v. 15, p. 1312-1320). The different roles played by each one of the paralogues have been studied.
In Hummel I. et al. (cf. Hummel et al., “Differential gene expression of ARGININE DECARBOXYLASE ADC1 and ADC2 in Arabidopsis thaliana: characterization of transcriptional regulation during seed germination and seedling development”, New Phytologist, 2004, v. 163, p. 519-531) the promoter activities of ADC1 and ADC2 were studied in stable transformants. In this report, it was found that chilling had a strong effect on ADC1 and ADC2 promoter activity. It was concluded that in Arabidopsis the polyamine response to chilling is shown to correlate with transcriptional activation of the ADC1 promoter.
In Alcazar et al. (Alcazar et al., “Overexpression of ADC2 in Arabidopsis induces dwarfism and late-flowering through GA deficiency”, The Plant Journal, 2005, v. 43, p. 425-436) an Arabidopsis transgenic plant was generated. The transgenic plant overexpressed the ADC2 gene, given rise to an accumulation of putrescine, without affecting the levels of spermidine or spermine. Furthermore, the plants overexpressing ADC2 showed dwarfism and late-flowering.
Despite the efforts made in the prior art, the research of new plants with improved stress resistance and methods for their obtention are still an active field.